Fluorescent lamps are typically constructed from a sealed glass tube that contains a small amount of mercury and an inert gas such as argon, xenon, neon, or krypton. Electrodes made of e.g., coiled tungsten are placed at different ends of the tube and are connected with an electrical circuit. When the mercury is properly vaporized within the lamp tube, applying a sufficient voltage difference across the electrodes will cause a current to flow through the gas in the tube thereby exciting the gas molecules and causing a release of photons—albeit in the form of short wave, ultraviolet light—a wavelength that does not provide the visible light that is desired.
The inside of the gas tube generally comprises a layer which includes phosphors—i.e. substances that can luminesce or give off light. More particularly, these phosphors are commonly applied as a paint-like coating to the inside of the tube. Organic solvents in the applied coating are allowed to evaporate leaving behind the phosphors. The tube may also be heated to remove residual solvent and fuse the coating to the lamp tube.
Photons released from the excited gas are absorbed by this coating of phosphors. In turn, the phosphors also emit photons but at a longer wave length than the photons released from the excited gas and, more importantly, at a wavelength that provides visible light. Variables such as the length of the glass tube determine how much visible light is provided by a particular lamp.
Advantageously, the fluorescent lamp converts the electrical energy supplied to its electrodes into a useful light more efficiently than a traditional incandescent lamp. In fact, much of the energy supplied to an incandescent lamp is lost in the form of heat. As a result, the fluorescent lamp is relatively less expensive to operate than an incandescent. Although the initial cost of a fluorescent is somewhat higher due to a ballast that is required in order to regulate the current, this cost is typically recovered in saved energy costs. Thus, the use of fluorescent lamps has become ubiquitous particularly in commercial applications.
For various reasons, after a period of use, fluorescent lamps eventually require replacement. By way of example, the electrodes may eventually fail, the small amount of mercury in the tube may absorb into the glass, the efficiency of the phosphors in absorbing and emitting photons may decrease, and other reasons may require replacement as well. As a result, a significant amount of fluorescent lamps must be disposed of each year.
The phosphors used in fluorescent lamps are typically rare earth compounds of various types. For example, europium-doped yttrium oxide (YEO) is widely used in fluorescent lamps as a red-emitting phosphor. Yttrium oxide that has been doped with other lanthanide series rare earth metals can also be used. A blend of phosphors, sometimes referred to as a triphosphor blend, is commonly used to provide white light from e.g., a red-emitting phosphor, a green-emitting phosphor, and a blue-emitting phosphor.
While the amount of phosphors used in producing an individual fluorescent lamp is relatively small, these phosphors are valuable materials. As such, recycling phosphors from discarded fluorescent lamps is desirable. Unfortunately, certain challenges are presented in attempting to reclaim these phosphor materials.
For example, the phosphors must be separated from multiple other materials that are used in the construction of the fluorescent lamps. These other materials can include glass used in constructing the tube, metals for the electrodes and other components, and plastics for parts such as a lamp base. In addition, certain organic materials may be present. These impurities may be introduced, for example, at some point between disposal of a spent lamp and recovery of the lamp from a waste disposal facility. The organic impurities can also result from the pyrolysis of a variety of materials during a mercury retorting heat treatment. Regardless, the organic impurities can discolor the phosphors causing them to exhibit e.g., a grey appearance and can also reduce the desired excitation by photons from the energized gas as discussed above.
One method for removing such organic impurities involves heating the recovered fluorescent lamp materials to a high temperature in the presence of a special atmosphere. Unfortunately, this process can be detrimental to some phosphors. Additionally, many of these organic materials may not be water soluble and, therefore, cannot be sufficiently removed by washing with water. Oxidizing acids or bases can be used but such can also result in phosphor damage and/or dissolution especially to yttrium europium oxide.
Accordingly, a process for the recycling of phosphors from fluorescent lamps would be useful. More particularly, a process for separating organic materials from the phosphors used in fluorescent lamps would be beneficial. Such a process that can be used with a variety of different phosphors and organic materials would also be particularly useful.